These studies show that the estimation of nephrogenous cyclic AMP reflects rapid changes in parathyroid activity and it can serve as an index of the entire spectrum of parathyroid function. In addition, nephrogenous cyclic AMP is increased in patients with "renal" hypercalciuria and normal volunteers taking furosemide 80 mg a day for 8 days, who develop secondary hyperparathyroidism. On the other hand, in "absorptive" hypercalciuria, nephrogenous cyclic AMP is normal, showing that the distinguishing feature is a normal or partly suppressed parathyroid function.